1. Field of the Invention
The invention relates to an intrinsically safe energy limiting circuit to be connected between an upstream DC power source and a downstream electrical load powered from that source.
2. Description of the Related Art
Intrinsic safety (IS) is a protection concept for safe operation of electrical equipment in explosive atmospheres and under irregular operating conditions. This concept, which is subject to a number of standards, relies on ensuring that the available electrical and thermal energy in the equipment is always low enough that ignition of the hazardous atmosphere cannot occur. In order to limit the electrical energy provided to the equipment, it is necessary to limit both current and voltage. The voltage is usually limited by a zener barrier. A series resistor between the zener barrier and the powered equipment provides instantaneous current limiting to prevent sparking or arcing when the current is interrupted. Furthermore, the current is limited to a safe value by a melting fuse between the power source and the zener barrier. As the input voltage provided by the power source is normally higher than the zener voltage, an additional resistor in series with the fuse is provided to prevent the fuse opening when exposed to a normal inrush current and to limit the continuous current through the zener barrier by dissipating the power imposed by the difference between the zener voltage and the input voltage.
From U.S. Pat. No. 7,852,610 B2 it is known to use a DC-to-DC converter to reduce the voltage provided by the upstream DC power source to a desired level for input into a conventional energy-limiting barrier. As there is no need for further voltage stabilization, the zener barrier of the energy-limiting barrier has a zener voltage higher than the nominal output voltage of the DC-to-DC converter.
As there is normally no additional current flow in the zener barrier and as modern switch-mode DC-to-DC converters have very low power conversion losses, it may be desirable to have two or more equipments (or more generally electrical loads) powered by the same upstream DC power source, each equipment or load receiving an individual output voltage from its own DC-to-DC converter and being protected by its own energy-limiting barrier. However, it may be a disadvantage that each conventional energy-limiting barrier has its own fuse because intrinsically safe fuses are expensive. Additionally, evaluation of the intrinsically safe circuit becomes more complex with two or more fuses, where the downstream devices are not separated electrically and the current is potentially shared between the fuses under normal or fault conditions. In this case, the current through both fuses may have to be combined as part of a fault scenario. This has the result of a difficult safety certification.
Another aspect is that intrinsic safety requires a certain distance between the two sides of a series protective element such as a fuse. The practicality of keeping both sides of the fuse separated in the middle of a circuitry is much harder than having the fuse up front in the physical layout of the circuit.